Your baby might be going through a growth spurts. Growth spurts usually takes place at 7-10days, 3 weeks, 6 weeks, 3 mths, 4 mths, 6 mths and 9 mths.
During the first 3 mths, when your baby hit growth spurts, they will cry for milk almost every hr. If you're breastfeeding, it is best to latch on to stimulate your breasts to produce more milk for their sudden growth.
This phase usually lasts about 1 week.